Recently, a quantum cascade laser, hereinafter abbreviated as “QCL”, is gathering much attention as a solid state light source that radiates, or emits, electromagnetic (EM) waves in the mid infrared (MIR) and the terahertz (THz) frequency ranges.
A typical QCL element is equipped with a semiconductor superlattice (SSL) which forms undulation of potentials for electrons inside, the potentials having repeating pairs of a well and a barrier. Upon application of an external voltage to the QCL element for operation, the potential for an electron in the SSL becomes inclined generally along a thickness direction while having the undulating pattern with the wells and barriers. The electron as a carrier of electric charge makes intersubband transitions repeatedly while being transported and emits EM waves through stimulated emission, leading to lasing operation. The expression “cascade” in the name has been given in association with the action of electrons, during which each electron loses its energy through such intersubband transitions while being transported. This action allows QCL elements to utilize their conduction carriers, or electrons, for stimulated emissions in a repeated manner, which is called “carrier recycling”. As stated above, the emission mechanism in QCLs is significantly different from one in conventional semiconductor lasers.
It is possible for QCL elements to cause lasing operation with a wavelength that has no relationship with an energy gap of the material used for the SSL. The lasing wavelength can be tuned through designing process of the SSL. For these reasons, QCL elements have attracted much attention as coherent light sources in the mid infrared and the THz ranges, for both of which wavelength or frequency ranges no solid state light source has been developed.
The EM waves in the THz range, among other things, have both properties of lights and radio waves. That is, EM waves in THz range indicate high resolution capability of lights and high transmission capability of radio waves, while adverse effects on target object are reduced in comparison with the case with X-ray radiation or the like. From this nature, the EM waves of the THz range are expected to be used in material identification through transmission and in fluoroscopic examination of human body. From a practical point of view, what is regarded as promising is QCLs with THz range radiation (THz-QCLs) that is capable of emitting radiation through the stimulated emission at the particular frequency as designed. Therefore, such THz-QCL elements need “band engineering”, in which thickness values of well and barrier layers in the SSL are carefully designed in consideration of, such as, the inclination of potential by electric field and detailed physical phenomena, and, on top of that, such SSL should be manufactured into actual devices. One measure to evaluate the practicality of THz-QCLs is an upper limit of temperature range in which lasing operation is carried out at intended operation frequencies (hereinafter referred to as “operation temperature”).
Elements of three-level type have been developed for the conventional QCL elements, including THz-QCLs as well as ones operating in the mid infrared range. In such type of QCLs, three electronic energy levels (hereinafter simply referred to as “levels”) for each stimulated emission in each repeating unit structure are related to the carrier transport and transitions. What is utilized in the three-level type QCLs is, in addition to the upper and lower lasing levels over which a population inversion is formed for lasing, a depopulation level with lower energy value than the lower lasing level. The depopulation level plays two roles: one is to depopulate electrons from the lower lasing level, and the other is to transport and inject electrons into an upper lasing level in the next stage, or downstream in the flow direction of the electrons. The electrons in the depopulation level are transported by tunneling of resonant tunneling effect when they are transported and injected into the upper lasing level in the unit structure downstream thereto. Such a mechanism is called resonant tunneling injection, or “RT injection”. To realize the RT injection it is necessary to match energy values with precision between a level, or a depopulation level in a unit structure, in which the stimulated emission in one stage occurs, and another level, or an upper lasing level in the unit structure next to it where the stimulated emission in the next stage occurs. Such an operation is achieved only when a limited and narrow condition is satisfied, thus it has been difficult to have a broad dynamic range in the current density in the three-level type QCLs that use the RT injection. In addition to that, when a large number of electrons exist in the upper lasing level in the next stage, a phenomenon called “carrier stagnation” may occur. The carrier stagnation is often observed when the population inversion is formed between the upper and lower lasing levels in the next stage.
The above-mentioned problems have been overcome for mid infrared range QCLs with four-level type (Patent Document 1: JP 2008-60396 A. In the four-level type, another level with higher energy value than the upper lasing level is used in addition to the three level type, which is referred to as an injection level in the Patent Document 1. Electrons that act as electric charge carriers will once make a transition to the injection level, and then, while being mediated by LO (longitudinal optical) phonons, will be injected into the upper lasing level with high rate. This scheme is called an indirect pumping (IDP) scheme. The indirect pumping scheme alleviates or removes the problems of the dynamic range and the carrier stagnation.